As a practical technique, vector-controlled (VC) strategy is implemented in induction motor drives (VCIMDs), serving various applications. These drives utilize voltage source inverters which are fed from conventional six-pulse diode bridge rectifiers. The most important drawback of these rectifiers is their poor power quality, i.e. current harmonics injection into ac grid. The circulation of current harmonics into the source impedance yields in voltage harmonics at the point of common coupling (PCC) and consequently undesired supply voltage conditions for nearby costumers.
The value of current harmonic components, which are injected into the grid by nonlinear loads, should be controlled within the standard limits. The most prominent standards in this field are IEEE standard 519 and the International Electro-technical Commission (IEC) standard.
For VCIMDs, one effective solution to eliminate harmonics is the application of the multi-pulse AC-DC converters. According to the recent investigations, these converters are based on either phase multiplication, phase shifting, pulse doubling or a combined solution (have been reported in U.S. patents such as U.S. Pat. No. 7,274,280, etc). Application of multi-pulse technique (up to 18-pulse) in AC-DC converters are reported in U.S. Pat. No. 7,375,996 where line current THD of more than 5% is experienced under different load conditions.
The polygon-connected autotransformer based 30-pulse (U.S. Pat. No. 7,719,858) was designed for AC-DC power converter. The DC link voltage in this topology is higher than that of a 6-pulse diode bridge rectifier, thus making the scheme non-applicable for retrofit applications.
The T-connected autotransformer based 40-pulse converter has been designed in (U.S. Pat. No. 8,982,595) for direct torque controlled induction motor drive (DTCIMD) with a current THD between 2.55% to 3.79% from full-load to light-load (20% of full-load), respectively. In some applications, it is necessary to take strict power quality measures; therefore, it would be essential to apply the converters with higher number of pulses. For instance, in some applications, harmonics are distinguished as signatures by sonar, and unintentionally are capacitively coupled to ship hull resulting in induced hull currents that make the systems such as degaussing equipment malfunction. In this situation, the operation of harmonic generating loads should be limited, in order to have a THD equal or less than 3%.